100 Mbit is what is called the wire-speed. We are talking packet switching here, and although once a packet is ready your interface ships it out at 100Mb/s, the processor is not capable of presenting data to the interface at a speed higher than (according to your remark) 266 KB per second (that's approx 1.2 Mbit/second I assume). Basically this means that although each packet is transmitted at the full wire speed, your packets are transmitted at such a pace, that the average throughput is stuck at 266 KByte/second...

Why 100 BT support? probably because more and more Interfaces are available at 100BT...
10BT, although certainly not dead (yet?) is slowly fading out (at least in the PC-environment it is...)